early 14c., bernak; earlier in Anglo-Latin, bernekke, early 13c., "species of northern European wild goose;" as a type of "shellfish" found in clusters on submerged wood, first recorded 1580s. Of unknown origin despite intense speculation.
The earliest form looks like "bare neck," and one of the Middle English synonyms was balled cote, but this might be folk etymology. The word is often said to be from a Celtic source (compare Breton bernik, a kind of shellfish), but the application to the goose predates that of the shellfish, and the word seems to have arisen in English.
The goose nests in the Arctic in summer and returns to Europe in the winter, hence the mystery surrounding its reproduction. It was believed in ancient superstition (and as recently as late 17c.) to hatch or develop from the barnacle's shell, possibly because the crustacean's feathery stalks resemble goose down. Some versions of the fable had the barnacles growing on trees and dropping into the sea to become geese. Compare German Entenmuschel "barnacle," literally "duck-mussel."
For I tolde hem, that in oure Countree weren Trees, that beren a Fruyt, that becomen Briddes fleeynge; and tho that fellen in the Water, lyven; and thei that fallen on the Erthe, dyen anon: and thei ben right gode to Mannes mete. And here of had thei als gret marvaylle, that sume of hem trowed it were an impossible thing to be. [Sir John Mandeville, "Voiage and Travaile," mid-14c.]
The scientific name of the crustacean, Cirripedes, is from Greek cirri "curls of hair" + pedes "feet." Meaning "person holding tenaciously to an office or position, useless or incompetent jobholder" is from c. 1600.